I don't know what to say... :o
Since I don't know much about how pianos work, I've done a little search.
This site seems quite good. The images are not large enough though to get the full grasp of what is happening.
Musical Services, Inc. - Piano Types
Oh, this site offers quite a lot to read:
Back check for piano - Patent 7169990
What I can see from the animation from the first site:
- There are moments involved (moments occur when you have long rigid object and a pivot, like a see-saw)
- For the horizontal piano (grand piano) gravity is definitively involved for the hammer to strike the string.
- What drives the hammer is the little 'L' shaped part, which also work on a system of moments.
I don't quite know what to talk about now... maybe someone can add?
What do you mean by rebuilding an acoustic piano? Repairing one which no more works or has defective keys?
If you knew what isn't working, you can easily replace the malfunctioning part I think... It's only my opinion, since I've got no experience with repairing pianos, nor tuning them.